1. Field of the Invention
This invention relates generally to a method and apparatus for reducing DC components in a digital information signal and, more particularly, is directed to a method and apparatus for reducing DC components in a digital information signal comprised of a plurality of N-bit data words by converting each of the N-bit data words to an M-bit data word.
2. Description of the Prior Art
It is known in the art of recording an information signal, such as a video or audio signal, on a recording medium, to convert the information signal in analog form to a digital information signal by means of pulse code modulation (PCM). The digital information signal is recorded on or reproduced from the recording medium, such as a magnetic tape, by rotating magnetic heads, and a rotary transformer is often used for supplying the digital information signal to or for receiving the reproduced information signal from the magnetic heads. However, the rotary transformer cannot transmit DC components of the digital information signal so that some of the information may be lost.
It has therefore been proposed to code the digital video signal to reduce the DC components thereof. In order to minimize the DC components of the digital information signal, the "0" and "1" bits of the digital signal are preferably arranged in a frequently occurring alternating sequence. Such alternation makes synchronous extraction of the signal during the reproduction operation easier, in addition to overcoming the above-described difficulties encountered with the rotary transformer. In other words, the separation of the information or data signals from the synchronizing signals during reproduction becomes exceedingly difficult if either of the "0" or "1" pulses is of a relatively long duration.
Further, it is desirable for the digital signal which is to be recorded to have the frequency spectrum thereof concentrated so as to prevent the occurrence of a so-called "peak shift" phenomenon. Such phenomenon results when a single-bit phase follows a relatively long duration pulse in the digital information signal whereby the signal corresponding to the single-bit pulse has its peak shifted during reproduction. It should be appreciated that the occurrence of such peak shift results in erroneous detection and reproduction of the recorded digital information signal. To avoid such "peak shift" phenomenon, it is desirable for the digital information signal to have frequent alternations of "1" and "0" bits. In other words, it is desirable that the levels of "1" and "0" have similar periods so as to concentrate the frequency spectrum of the digital information signal.
In order to minimize the DC components and concentrate the frequency spectrum of the digital information signal, various code conversion systems have been proposed which, for example, convert a digital signal comprised of 8-bit words to a digital signal comprised of a 10-bit words. The 8-bit word code is comprised of 2.sup.8 or 256 possible 8-bit words and the 10-bit word code is comprised of 2.sup.10 or 1,024 possible 10-bit words. In order to maximize the frequency of alternation of the "1" and "0" bits, known 8-to-10 code conversion systems have primarily used 10-bit words having five "0" bits and five "1" bits in the conversion process. However, of the possible 1,024 possible combinations, there exist only 252 10-bit words having five "0" bits and five "1" bits. Accordingly, at least four 10-bit words in which the difference between the number of "1" bits and "0" bits is two, for example, the words [1010101011] and [0101010100], must be added to the aforementioned set of 252 10-bit words during the coding process to complete the code map for all 256 combinations of the 8-bit word code.
However, although known 8-to-10 code conversion systems at least partially remove the DC components from the digitial information signal, such systems do not take into consideration the degree of concentration of the frequency spectrum of the digital information signal. For example, of the 256 possible 10-bit words in the cope map, there are included words of a relatively long-pulse duration [1111100000], that is, words having a long run length, in addition to words of a relatively short-pulse duration [1010101010] having a short run length. This means that a short-pulse duration word may follow a long-pulse duration word whereby to produce the aforementioned peak shift phenomenon, resulting in errors in the reproduced digital information signal.